1. Field of the invention
This invention relates to a semiconductor laser device with a lengthened life span.
2. Description of the prior art:
Assurance of high reliability has already been reached when optical output of GaAlAs semiconductor laser devices with an oscillation wavelength, for example, in the 780 nm region, is at a low level of 5 mW or less. Such laser devices are widely used as a light source in optical information processing systems such as those used for digital audio discs, laser discs, etc. However, in recent years, semiconductor lasers with high optical output, 30 mW or more, are in great demand as a light source for writable laser discs. The most widely used method to achieve high output of semiconductor laser devices is by creating non-symmetric reflectivity of the protective films of the facets. That is, in this method, the front facet is covered with a dielectric film of low reflectivity and the back facet is covered with a dielectric film of high reflectivity, thereby improving the differential efficiency and raising the kink level. In practice, the front facet is covered with a protective film of Al.sub.2 O.sub.3, Si.sub.2 N.sub.4 or the like with a thickness of .lambda./4n (wherein .lambda. is the oscillation wavelength and n is the refractive index of the film) by electron-beam vapor deposition, sputtering deposition, plasma chemical vapor deposition, etc. The back facet is covered with a protective film composed of alternate layers for a total of four layers of either Al.sub.2 O.sub.3 or Si.sub.2 N.sub.4, and a-Si:H.sub.2 with a thickness of .lambda./4n each in the same manner as in the front facet. As a result, the reflectivity of the protective film of the front is about 6%, and the reflectivity of the protective film of the back is about 90%. Thus, compared to widely used Al.sub.2 O.sub.3 or Si.sub.2 N.sub.4 single layer protective films with a thickness of .lambda./2n which are used on both facets, the differential efficiency and the kink level are both improved two-fold or more, and the semiconductor lasers can achieve a high output power.
However, high reliability in practice is not always achieved for the reasons mentioned below: In order to observe the degree of deterioration of semiconductor laser devices with a protective film on the facets made in the above-mentioned manner, accelerated life tests were carried out at an atmospheric temperature of 50.degree. C. on these semiconductor laser devices with the optical output fixed at 30 mW, the driving current which is indicated by the curves shown in FIG. 3(a) increases almost linearly with the driving time, and at times tends to increase rapidly with the lapse of time.
These deteriorated semiconductor laser devices were then subjected to a deterioration-analysis using the EBIC method by the inventors of the present invention, and it was found that only the striped region in the vicinity of the front facet covered with only one layer of a protective film of Al.sub.2 O.sub.3 with a thickness of .lambda./4n was deteriorated so that the intensity of the EBIC image at the region was weaker than that of the other region. Moreover, it was found that the back facet covered with alternate layers consisting of four layers of Al.sub.2 O.sub.3 and a-Si:H.sub.2 was not deteriorated. This deterioration pattern is of the same type as that of oxidation deterioration that occurs when the facets are not covered with a protective film, so that, during high output operation, the optical density of the inside of the laser devices becomes high and a large amount of current is injected thereinto, resulting in oxidation deterioration of the facet. Therefore, the formation of only one layer of a protective film on the facets cannot attain the suppression of oxidation of the facets. This tendency is also present when Si.sub.2 N.sub.4 is used as the protective film. On the other hand, the inventors have found that a multi-layered film of Al.sub.2 O.sub.3 and a-Si:H.sub.2 is effective not only to make reflectivity high but also to suppress oxidation.